Conventionally, in a linear motion guide unit, in order for rolling elements to smoothly enter or exit from a load-carrying race formed by a raceway groove of a carriage of a slider, gently sloped crowning portions are formed at opposite ends of the raceway groove of the carriage, which opposite ends are inlet portions of the load-carrying race. In a conventional linear motion guide unit, for example, a load-carrying race is composed of inlet portions and a substantially load-carrying portion extending between the inlet portions. The conventional linear motion guide unit has the carriage in which raceway grooves are formed so as to serve as load-carrying races, and end caps which are disposed at opposite ends of the carriage and in which respective turnaround passages are formed. The crowning portions are gently sloping surfaces formed respectively at opposite ends of the raceway groove of the carriage. At a connection end of the carriage between the load-carrying race and the turnaround passage, an end surface of the carriage where the crowning portion merges into the inner circumference of the turnaround passage is chamfered; in other words, an end of the crowning portion is chamfered.
Japanese Patent Application Laid-Open (kokai) No. 2002-155936 discloses a guide unit in which in order to achieve a high-speed movement of a slider, there is eliminated the presence of any jagged structure against rolling elements which roll from a no-load area to a load area, whereby sliding resistance and noise are reduced in high-speed relative movement between the slider and a guide rail. In the guide unit, a load-carrying race has crowning areas at respective opposite ends of the load area of the slider; specifically, as the load-carrying race approaches a turnaround passage, the race gradually expands such that an edge portion of the load-carrying surface is recessed in relation to the inner side wall surface of the turnaround passage to thereby form a step portion. As a result, even in the presence of an error in forming a guide portion and an error in forming the load-carrying race, the load carrying race does not protrude toward the guide rail from the side wall surface of the turnaround passage. The size of the step portion is about 5% of the roller diameter.
Japanese Patent Application Laid-Open (kokai) No. 2003-269450 discloses a linear motion unit configured to enable smooth circulation of rollers by eliminating the influence of a certain manufacturing error at connections between roller passages which constitute a roller circulation passage. In the linear motion unit, a sloped portion is formed at a connection between a roller rolling passage composed of a guide groove of a guide rail and a guide groove of a slider body, and a roller-return-passage curved portion defined by a return guide and an end cap attached to the slider body. The sloped portion expands in diameter from the roller rolling passage toward the roller-return-passage curved portion. In the linear motion unit, an end portion of the slider body has an inclined inner circumferential surface; an end portion of an engagement protrusion of the return guide has an inclined outer circumferential surface; the inclined outer circumferential surface of the return guide is fitted into the inclined inner circumferential surface of the end portion of the slider body; and as a result of the end portion of the engagement protrusion of the return guide having the inclined outer circumferential surface, the end portion of the engagement protrusion is thin-walled.
Japanese Patent Application Laid-Open (kokai) No. 2008-133837 discloses a linear motion guide unit which exhibits improved durability in high-speed running and restrains deterioration in load capacity and rigidity through prevention of worsening of vibration resulting from passage of rolling elements. In the linear motion guide unit, each of sloped portions provided at respective opposite ends of a slider-side rolling-element raceway groove is composed of a first crowning portion in the form of a curved surface having such a large radius of curvature as to be gently sloped continuously from the slider-side rolling-element raceway groove, a planar second crowning portion adjacent to the first crowning portion, extending toward the inner circumferential surface of a turnaround passage, and shorter than the first crowning portion, and a sloped surface provided between the second crowning portion and an end surface of a slider body and sloped more steeply than the first and second crowning portions.
Japanese Patent Application Laid-Open (kokai) No. 2005-273765 of a patent application filed by the applicant of the present invention discloses a linear motion guide unit having standard specifications for allowing use in wide applications, allowing easy formation of crowning portions, having a relatively small crowning depth, and exhibiting high precision and durability. In the linear motion guide unit, at gateways to a raceway groove of a carriage, which gateways partially constitute a load-carrying race, there are formed respective crowning portions having the form of gently curved surfaces for allowing rolling elements to smoothly enter and exit from the load-carrying race, and radiused portions in the form of curved surfaces formed at respective opposite ends of the raceway groove. The crowning portions and the corresponding radiused portions are connected continuously and smoothly.
Meanwhile, in recent years, demand has been rising for a high-speed linear motion guide unit. Due to high-speed rolling of rolling elements, the high-speed linear motion guide unit involves, for example, the following problem: at a connection between a carriage and each end cap, rolling elements hit against an inner surface of a load-carrying race at an end portion of the carriage; as a result, exfoliation or flaking occurs at the end portion of the carriage. Specifically, in a linear motion guide unit of a retainer-less type, in the course of movement of rollers, or rolling elements, from a turnaround passage to a load-carrying race, when a roller passes the tip of a scooping beak of an end cap body and an end of a raceway groove of a guide rail; i.e., an end of a raceway surface of the guide rail, the roller collides with the guide rail and bounds, and the bounding roller collides with an opposite corner portion of an end of the carriage, potentially resulting in exfoliation; i.e., flaking, of the corner portion.
In view of the foregoing, the inventors of the present invention studied feasibility of the following means to avoid exfoliation of or damage to a corner portion of the carriage in the linear motion guide unit: an extending portion which protrudes toward the carriage side from a spacer of an end cap at a position adjacent to the inner circumferential surface of the spacer, which partially constitutes a turnaround passage, is formed in such a manner as to be flush with a raceway groove of the carriage, whereby the bounding roller or rolling element collides with the extending portion, and thus the extending portion absorbs impact force of the colliding roller to thereby avoid exfoliation of or damage to the corner portion of the carriage.